We've come a long way since Hank Williams and Robert Johnson sat down and poured their heart into a song, aided by a trusty guitar and a rusty microphone. But even if popular music is much changed, the cult of the singer-songwriter remains intact. It's not just Mojo readers who subscribe to it; most of us would agree there is a credibility gap between a performer who is spoonfed material and a songwriter.

If you're a proper songwriter, the logic goes, you don't need hot producers, slick videos or an army of image consultants (does Neil Young have a stylist?) - you just get onstage and do it.

That's why Beck, the smartest chameleon in pop, has ditched the smoke and mirrors, along with The Flaming Lips who accompanied him on his American tour last year, for a round of solo European shows.

Throughout his career, Beck has experimented with blues and folk, occasionally playing tiny venues and incorporating acoustic sections into his performances. But his latest album, Sea Change, alternately 'the best break-up album since Dylan's Blood on the Tracks ' or 'a bit of a downer', has given him the opportunity to become Beck Unplugged.

But while the downbeat, heartfelt songs on Sea Change mark a change from Beck's oblique lyrics, the production values, straightforward ballads awash in swooping string arrangements, are impressive. Not quite back to basics, then.

The art-deco Grand Rex cinema is a fittingly theatrical venue for Beck's Parisian gig, while the audience look as if they trooped off the pages of a style magazine - plenty of flared and faded denim and geometric haircuts.

The stage is unadorned, except for a line of acoustic guitars, an organ and an upright piano that has clearly seen better days. Beck appears in a skinny black suit and black shirt divided by a large studded belt, although any hint of rock'n'roll menace dissipates when you take in his slight figure and Peter Pan face.

Tonight is the ultimate test of Beck the songwriter, and he passes - though not quite with flying colours. Like most people, I'd rather see Beck having fun, sliding across the stage on his knees, wielding his guitar like a weapon of mass seduction and imitating Prince's lascivious yelps, all the while with a knowing smile.

However, within a few bars of the opener, 'Guess I'm Doing Fine', the audience stirs. The postmodern imp has turned into a lonesome troubadour. When he sings: 'It's only lies that I'm living/ It's only tears that I'm crying/ It's only you that I'm losing', he really means it.

His wounded baritone is not the most versatile instrument in pop but there is no doubting the passion he conveys. Has he ever laid himself bare like this on stage before? As he keeps a poker face and strums like a man possessed, I quickly warm to this new, grown-up Beck. Other highlights from the new album include 'The Golden Age', in which the world-weary singer manages to sound like a 70-year-old busker and 'Already Dead', a heartbreaker of a song about the bitter end of a relationship. There are a couple of missteps, notably when he does 'Cold Brains' backed by an accordion.

Still, Beck being Beck, he just can't resist showing off. He plays half-a-dozen different guitars, including some virtuoso slide blues on a National Steel; he is pretty useful on piano and harmonica, too. He programmes a drum machine on 'Nicotine & Gravy' and 'Tropicalia', messing about with tempo and rhythm patterns midsong just for the hell of it.

For 'Debra', he thumps away on an organ and lets fly with a series of outrageous falsetto cries that remind us that we shouldn't take him too seriously. Then he moves centrestage and transforms 'Loser' from a folky anthem into a swaggering piece of garage rock that would make Jack White swoon.

He is even open to requests. When a persistent fans demands 'The Mexican', he not only plays it, he invites the fan down from the balcony for an impromptu duet. The result is pretty good and the song gets the biggest cheer of the night. In fact, it's a better collaboration than the one he has with local boys Air who join him for a lacklustre encore.

But by the time he appears for the third encore stamping his feet and playing harmonica, he has whipped the fashionable crowd into a frenzy.

It's a brave performance, which proves that even without the tricks, Beck is still a force. For anyone who thinks he has been treading water since Odelay , this is a reminder that he is putting together a substantial body of work.

I've had more fun at Beck gigs before, but this is the most surprising and moving performance I've seen in a long time.